In recent years, there is demanded a method for effectively utilizing heat energy and saving energy. As such a method, a method has been proposed in which heat is stored, employing latent heat associated with phase change of a material. As compared with a method employing only sensible heat which is not associated with phase change, this method has advantages in that a heat storage material volume can be reduced because a large quantity of heat energy can be stored with high density in a narrow temperature region including a melting point and heat loss can be suppressed because no large temperature difference occurs for a large quantity of storage heat.
In order to increase heat exchange efficiency of a heat storage material, a method has been proposed which microencapsulates the heat storage material. As a method to microencapsulate a heat storage material, there are an encapsulation method according to a composite emulsion method (for example, refer to Japanese Patent O.P.I. Publication No. 62-1452), a method which forms a thermoplastic resin layer on the surface of heat storage material particles in a liquid (for example, refer to Japanese Patent O.P.I. Publication No. 62-149334), a method which forms a polymer layer derived from polymerization of a monomer on the surface of heat storage material particles (for example, refer to Japanese Patent O.P.I. Publication No. 62-225241), and a method which manufactures microcapsules having on the surface a polyamide layer formed according to interface polymerization (for example, refer to Japanese Patent O.P.I. Publication No. 2-258052).
In most of the microencapsulating methods above, heat storage material microcapsules are dispersed in a medium to form a heat storage material microcapsule dispersion. The dispersion makes it possible to easily transport in various apparatus. Further, the dispersed microcapsules in the dispersion can be dried and extracted as a solid component, whereby the heat storage material can be kept in the solid state whatever state it may originally be.
As a phase change heat storage material, aliphatic hydrocarbon compounds such as tetradecane and hexadecane are employed. These compounds melt or solidify at a relatively low temperature, and are suitable to micro-encapsulation described above. However, careful attention must be paid to use these, since these have a small latent heat amount and are likely to catch fire.
In contrast, an inorganic salt hydrate and sugar alcohol are preferably employed as a heat storage material, since they have a large latent heat amount and are difficult to catch fire at high temperature. The inorganic salt hydrate and sugar alcohol are water-soluble, and therefore, they need to be microencapsulated according to a method in which they are incorporated in a water immiscible organic solvent to form a W/O emulsion and then the surface of the resulting emulsified particles is covered with a hydrophobic resin layer. However, this method is difficult to form a stable emulsion, and therefore, it is difficult that the water-soluble latent heat storage material is completely encapsulated. Further, this method has problem in durability, that is, the inorganic salt hydrate, when subjected to repeated heating and cooling cycles, causes lowering of latent heat due to phase separation, and the latent heat storage material causes lowering of the quantity of latent storage heat due to supercooling phenomenon.